Ionic conductivity is usually associated with the flow of ions through an aqueous solution of metallic salts. In the vast majority of practical uses of ionic conductors, e.g., as electrolytes for dry cell batteries, the aqueous solution is immobilized in a paste or gelled matrix to overcome the difficulties associated with handling and packaging a liquid. However, even after immobilization, the system is still subject to possible leakage, has a limited shelf life due to drying out or crystallization of the salts and is suitable for use only within a limited temperature range corresponding to the liquid range of the electrolyte. In addition, the necessity of including a large volume of immobilizing material has hindered the aims of miniaturization.
In attempting to overcome the shortcomings of liquid systems, investigators have surveyed a large number of solid compounds hoping to find compounds which are solid at room temperature and have ionic conductances approaching those exhibited by the commonly used liquid systems. Such compounds have specific conductances at room temperature (20.degree. C.) in the range of 10.sup.-6 to 10.sup.-15 ohm.sup.-1 cm..sup.-1 as compared to aqueous solutions of salts which typically have a specific conductance of 0.5 to 0.05 ohm.sup.-1 cm.sup.-1.
Improved microelectronic circuit designs have generally decreased the current requirements for electronic devices. This in turn has enhanced the applicability of solid electrolyte power sources which usually can only deliver currents in the microampere range. These solid electrolyte systems have the inherent advantages of being free of electrolyte leakage and internal gassing problems due to the absence of a liquid phase and corrosion phenomena. In addition, they also have a much longer shelf life than the conventional liquid electrolyte power sources.
Solid electrolytes must be essentially electronic insulators so as not to internally short the cell while at the same time they must allow ionic movement in the crystal lattice for the cell to operate. It has been discovered that certain metallic salts which are solids at room temperatures have ionic conductances sufficiently high to permit their use in practical battery applications. For example, U.S. Pat. No. 3,723,185 discloses solid state electrolytes of compounds conforming to the general formula AgI-MCN-AgCN or modifications thereof wherein M is potassium, rubidium, cesium or mixtures thereof.
U.S. application Ser. No. 973,554 filed Dec. 27, 1978 titled Solid State Electrolyte, discloses a relatively highly conducting solid state electrolyte made from a binary mixture of lead fluoride and a 0.1 to 15.0 mole percent of a selected alkali salt of sodium, potassium, rubidium and cesium. U.S. application Ser. No. 973,552 filed Dec. 27, 1978 titled Solid State Electrolytes discloses a relatively highly conducting solid state electrolyte of a product of a fused ternary mixture of lead fluoride, an alkali salt of sodium, potassium, rubidium or cesium in a range of 0.1 to 15.0 mole percent and a fluoride, nitrate or sulfate of magnesium, calcium, strontium or barium in a range of between 1 and 40 mole percent. Both of these U.S. applications are incorporated herein by reference.
Although various solid cathode materials are recited in the art for use in various cell systems, an object of the present invention is to provide a novel cathode for use in solid electrolyte cell systems employing solid electrolytes such as those disclosed in the above-identified U.S. patent applications Ser. No. 973,552 and No. 973,554.
Another object of the present invention is to provide a solid cathode for use with a solid fluoride-containing electrolyte wherein said cathode comprises a mixture of manganese dioxide and a metal fluoride.
Another object of the present invention is to provide a cathode consisting essentially of manganese dioxide and a metal fluoride for use in a lead flouride-containing solid electrolyte system, and wherein the open circuit voltage of the system is higher than the open circuit voltage obtained using either manganese dioxide or a metal fluoride alone as the cathode in the system.
Another object of the present invention is to provide a cathode consisting of manganese dioxide and a metal fluoride for use in a solid electrolyte cell employing a lead anode and a lead fluoride-based solid electrolyte as disclosed in U.S. applications Ser. No. 973,552 and No. 973,554.
Another object of the present invention is to provide a cathode consisting of a mixture of manganese dioxide and a metal fluoride in a mole ratio of between about 1:2.5 and about 3:1.